238 DR. K. H. THOMAS ON XENOLITHIC [vol. Ixxviii, 



can be fairly sure that their intrusion-temperature was not particu- 

 larly high. The tholeiite of Rudh' a'Chromain, however, at its 

 junction with the Carsaig Sandstone, has for a few millimetres 

 thermally altered the sedimentary rock. The action has been to 

 produce an interstitial melt between the component grains of the 

 sandstone, which has attacked and dissolved the quartz, with 

 the subsequent crystallization of tridymite -fringes around the 

 undissolved portions of the grains. The tridymite has reverted to 

 quartz which, while being in optical continuity with the quartz 

 of the original grain, retains the form of tridymite. This pheno- 

 menon is much better displayed by the quartz-grains in the 

 siliceous xenoliths, the metamorphism of which is of a more 

 intense character (see p. 240). 



III. The Coghstate Xenoliths (Enclaves Homceoq^nes). 



Cognate xenoliths are a striking feature of the basic portions of 

 the Rudh' a'Chromain and Nuns' Pass sill, and of the tholeiitic 

 intrusions of several other localities under consideration. Such 

 xenoliths were first described from the tholeiites of this region by 

 Dr. Flett, who recognized two types : one consisting of bytownite 

 and pale-green augite, and the other of bj^townite and a nearly 

 colourless enstatite or bronzite.^ 



In the Hudh' a'Chromain sill they occur as dark coarsely-crys- 

 talline patches, sharply marked ofE from the more finely-grained 

 tholeiite that envelops them. As was noted in the similar occur- 

 rence of Traigh Bhan na Sgurra, they show a distinct tendency to 

 congregate in the lower portion of the sill, suggesting a gravita- 

 tional concentration. They consist commonly of b^^townite and 

 hypersthene. The hypersthene-crystals are often sharply idio- 

 morphic (PI. VI, fig. 6) and have moulded upon them, or occa- 

 sionally include, an optically positive basic plagioclase (basic 

 labradorite or bytownite). 



Less frequently a dark-green augite takes the place of the 

 rhombic pyroxene (PI. YI, fig. 5), and like it is usually idiomorphic 

 towards the bytownite (16612^, 17173 ).2 In some cases olivine 

 appears to have been present, but is now represented by serpen- 

 tinous and calcitic pseudomorphs (17174). An orthorhombic and 

 a monoclinic pyroxene are sometimes, though rarely, associated in 

 the same xenolith. 



Hypersthene, presumably derived from cognate xenoliths, occurs 

 in the intrusion of Mullach Clac an t'Sneachda (Locality 2), while 

 coarse aggregates of bytownite and augite have been met with at 

 Seabank Villa (Locality 14), where some of the augite-crystals 

 are sharply idiomorphic (18530), and occasionally measure nearly 

 2 inches in length. 



^ ' G-eology of Colonsay & Oronsay, witli part of the Eoss of Mull' Mem. 

 Geol. Surv. 1911, p. 92. 



2 These numerals in parentheses refer to the rock-slices in the collections 

 of the Geological Survey. 



